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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Causes of Cyclic Shear Failure at Lokanthali of Araniko Highway after Mw7.8 2015 Gorkha Earthquake Binod Tiwari, Ph.D., P.E.1 Beena Ajmera, Ph.D.2 , and Brian Yamashiro3 1Professor, Civil and Environmental Engineering Department, California State University, Fullerton, 800 N State College Blvd., E-419, Fullerton, CA, 92831. 2Assistant Professor, Civil and Environmental Engineering Department, California State University, Fullerton, 800 N State College Blvd., E-318, Fullerton, CA, 92831. 3Graduate Student, Civil and Environmental Engineering Department, California State University, Fullerton, 800 N State College Blvd., Fullerton, CA, 92831. ABSTRACT The Mw 7.8 2015 Gorkha Nepal Earthquake was one among the most devastating earthquakes in in the history. It left close to 9,000 people dead, 22,000 people injured and caused over $5B loss of properties. The earthquake triggered over 15,000 co-seismic landslides. Although the major highways and bridges performed well, majority of the hydropower projects located within the earthquake-affected area sustained different types of damages. A few dozens buildings, including several modern constructions, collapsed in Kathmandu due to the earthquake shaking, killing thousands of people. The earthquake also caused cyclic shear failure of a sector of Araniko Highway at Lokanthali, Kathmandu. This highway is very important trade route to China. A 250 m long stretch of the highway collapsed with vertical movements of more than a meter and lateral displacements of approximately 0.5 m. The landslide seriously damaged dozens of buildings and displaced a few of them by approximately 1 m. To investigate the causes of ground failure, the authors performed site investigation including subsoil exploration, in-situ tests and laboratory tests on the slope forming materials in addition to topographic mapping. Additionally, deformation and limit equilibrium analyses were also performed using the laboratory and field data. The study revealed that earthquake induced shaking in combination with post-cyclic strength degradation of the lacustrine deposit, locally named Kalimati, caused the cyclic shear failure although the site has gentle slopes and the earthquake main shock produced relatively moderate ground accelerations at this location. INTRODUCTION The Mw = 7.8 2015 Gorkha earthquake was one among the deadliest earthquakes in the South Asian region. It killed close to 9,000 people and injured more than 22,000, displaced millions of residents, and left hundreds of thousands of people homeless. Estimated economic loss resulted due to the earthquake is over $5B (almost 25% of the Nepalese GDP). The epicenter of the Gorkha earthquake is located at Barpak, Gorkha, approximately 80 km northwest of Kathmandu. The hypocenter of the earthquake is estimated at a shallow depth of 15 km (Hashash et al., 2015). The first author co-lead the NSF funded 15-member GEER team for post-earthquake reconnaissance immediately after the earthquake and spent 30 days in the field to collect perishable geotechnical information. Despite the large magnitude of earthquake, the observed peak ground accelerations, recorded at very few locations in and around Kathmandu valley were relatively small. As such, the damage caused by this earthquake was much less than expected for an earthquake with this magnitude. However, at clustered locations, significant damage was observed. There was very limited ground motion information that could be useful to analyze and assess the individual damage. One among those limited information was the ground motions (Figure 1) recorded in Kathmandu Valley at the KATNP station, which is located about 6.3 km north-west from the site. USGS released the ground motion data recorded at this station immediately after the earthquake. Both the north-south and east-west components of the peak ground accelerations (PGA) recorded at this station were surprisingly low, at the order of 0.16g. Dixit et al. (2015) analyzed the records from the main shock, as well as additional recordings from aftershocks and microtremors, and reported a dominant period of about 5 seconds for the main shock. Although majority of the highways and bridges performed well during and immediately after the earthquake, several mountain roads including the access roads to hydropower projects were fully damaged at hundreds of locations, completing blocking the through traffic. One among the spectacular damage along the roads in Kathmandu Valley was Lokanthali Cyclic shear failure. This road was widely covered in media and reconnaissance reports. The GEER team mapped the damage, performed open cuts for soil exploration, and performed a few laboratory experiments on the field samples collected during the field visit (Hashash et al. 2015; Moss et al. 2015). The first author made three more trips to Nepal in addition to the month-long field reconnaissance with the GEER team. The latest trip was in August 2016, to collect post- earthquake field information. Detailed topographic survey, in-situ soil tests, and soil sampling were performed at Lokanthali cyclic shear failure area to evaluate the cause of failure. The subsequent section includes details of those investigation and the results of stability and deformation analyses performed. EXTENT OF DAMAGE The 2015 Gorkha Earth caused a significant damage to building structures in Kathmandu and other moderately large towns located in the eastern side of the main shock. Although the level of damage was much less than expected with such large magnitude earthquakes, a few dozen of modern structures were fully or partially damaged, killing hundreds of people (Figure 2). Moreover, majority of the other damaged buildings were constructed with brick masonry structures on mud mortar. Aerial reconnaissance with helicopter revealed that hundreds of buildings located on the ridge of the slopes within the rupture surface were fully collapsed. Dozens of cultural heritage sites and temples were either fully or partially damaged due to the earthquake induced ground shaking. Figure 1. Ground accelerations recorded at the KATNP station during the April 25, 2015 Gorkha earthquake (CESMD, 2015). The 2015 Gorkha earthquake triggered liquefaction at more than 15 locations (Figure 3). At those locations, traces of liquefied sand ejecta were observed. Some of those ejecta were in significantly large in quantity and the others were clearly visible traces, but not in abundant quantity. The field observation showed that the earthquake shaking was just enough to liquefy the soil, but was not sufficient to cause a massive damage to structures. Figure 2. Extent of structural damage observed in Kathmandu Valley due to the 2015 Gorkha Earthquake. Majority of the damage were soft-story collapses. The 2015 Gorkha earthquake significantly affected the hydropower projects. While majority of the damage was observed in the dams, penstocks and powerhouses due to landslides and rock falls, 19 cm of seismically induced settlement was observed on the dam body of the Upper Tamakoshi Hydropower project, the largest Hydropower Project of the nation, which was under construction at the time of earthquake (Figure 4). As expected, the 2015 Gorkha earthquake triggered approximately 15,000 co-seismic landslides (Figure 5). Approximately 3,500 of these landslides were larger than 100 m2 in area. Tiwari et al. (2016) correlated those landslides with terrain slope, geology, and peak ground acceleration (PGA) and reported that slope inclination and PGA were the most influencing factor in triggering those landslides. More than 99% of those triggered landslides were disruptive landslides. Although very small in number, the coherent landslides triggered by the earthquake caused significant damage to structures and killed hundreds of people. Two among those landslides are Langtang Debris Avalanche and Lokanthali cyclic shear failure. Description of Langtang Debris Anavalche and other valley blocking landslides are available in Hashash et al. (2015), Moss et al. (2015) and Collins and Jibson (2015). This article discusses in detail about the Lokanthali cyclic shear failure site. Figure 3. Sand ejecta observed at the edges of Kathmandu Valley due to liquefaction. The liquefaction sites, although were wide spread, did not damage structures. Figure 4. 19 cm settlement observed at the main dam of Upper Tamakoshi Hydropower project due to seismically induced settlement of the foundation material. Lokanthali cyclic failure site is located about 1 km east of Kathmandu airport in Nepal (27°40’28.1”N, 85°21’44.6”E). Hashash et al. (2015) and Moss et al. (2015) mapped this failure site. Field evidence of ground failure was observed along a ridge for a distance of about 1 km. At different locations, fissures larger than 2 m and tension cracks with vertical offsets larger than 1.4 m along the top of the ridge, were observed (Figure 6). The surprising feature of this cyclic failure was that the ground slope was very gentle and such failure was not expected at this newly renovated highway. Figure 5. Typical co-seismic